Fasting during the month of Muharram, especially the day of 'Ashurah
When the Day of Judgment comes, we will without doubt all desire to steer clear from our sins and wish that we had done more good deeds that would be a cause for removal of our bad deeds.
Of much interest to us Muslims
these days should be the fasting the 9th and the 10th of Muharram. The Prophet,
sallallahu 'alayhi wa sallam, said about the voluntary fasting on the Day of
"It expiates the sins of the preceding year." [Saheeh Muslim]
radhiallahu 'anhu, reported:
"The Prophet, sallallahu 'alayhi wa sallam, came to al-Madeenah and saw the Jews fasting on the day of 'Aashooraa. He asked them about that. They replied, "This is a good day, the day on which Allah rescued Baani Israaeel from their enemy. So Moosa observed fast on this day." The Prophet (pbuh) said, "We have more claim over Moosa than you." So the Prophet, sallallahu 'alayhi wa sallam, observed fast on that day and ordered (the Muslims) to observe fast (on that day)."
[Saheeh al-Bukhari, Muslim]
Ibn 'Abbas also reported, "The Messenger of Allah fasted on the day of 'Aashooraa and ordered the people to fast on it. The people said, "O Messenger of Allah, it is a day that the Jews and Christians honor." The Prophet, sallallahu 'alayhi wa sallam, said, "When the following year comes - Allah willing - we shall fast on the ninth." The death of the Prophet, sallallahu 'alayhi wa sallam, came before the following year." [Saheeh Muslim, Abu Dawood]
So it is recommended to fast on the 10th of Muharram and the 9th along with it as well.
Qadi ibn Rushd says in that the scholars disagreed whether the fast is to be done on the 9th or the 10th of Muharram based on the above traditions [Bidayat ul Mujtahid]. And I have found a recommendation to fast on both days in various books, such as al-Hindees "Mukhtasar al-Ahkaam al-Fiqhiyyah", Jameel Zenos "The Pillars of Islam and Iman", and Dr. Jaafar Sheikh Idris "The Fast."
As for the wisdom behind this fast, the following is stated in the mukhtasar version of Ibn ul Qayyims "Zaad ul-Ma'aad" "The day of 'Aashooraa was the day when Moosa and his men got freedom from the hands of the Pharaoh and it was in the sacred memory of this great event that the Muslims observed voluntary fasting. The idea underlying this is to stress the affinity amongst the messengers of Allah and to show that religious devotion is a constant flow from one generation to another. The Prophet, sallallahu 'alayhi wa sallam, came not to abrogate all the earlier religious practices but to codify and preserve them for all times to come in ideal forms."